As a result, they were looking for images of someone lugging a dark, heavy bag. Investigators had appealed to the public to provide videos and photographs from the race finish line.

One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off," Murphy said. He said he was briefed by Boston police.

Separately, a law enforcement official who was not authorized to discuss the case publicly and spoke to The Associated Press on the condition of anonymity confirmed only that investigators had an image of a potential suspect whose name was not known to them and who had not been questioned.

Several media outlets reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the bomb blasts.

Seven bombing victims remained in critical condition.

Dr. Peter Burke, chief of trauma surgery at Boston Medical Center, said Thursday that one of the youngest victims, a 5-year-old boy is getting better and "is going to be OK." A blast can often compress a child's chest, bruising the lungs and heart, he said, adding he is pleased with the boy's progress.

Dozens of victims have been released from hospitals, and officials at three hospitals that treated some of the most seriously injured said they expected all their remaining patients to survive.

The blasts killed 8-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Medford, and Lu Lingzi, a Boston University graduate student from China.